Gene Transfer into Extremophilic Eukaryote
Extremophilic Eukaryote: Galdieria sulphuraria The unicellular G. sulphuraria leads a life most unusual for a single-celled eukaryote by thriving in acidic, hot springs. G. sulphuraria possess the extraordinary ability to metabolism a wide variety of carbohydrates, perform photosynthesis, and can also survive in environments marked by metal-rich, high salt, and arsenic concentrations. These extremophilic and optional heterotrophic and photoautotrophic traits can be traced back to the G. sulphuraria genome. The G. sulphuraria genome shares remarkable similarity to bacteria and archaea, by sharing 75 genes with both of these seemingly distant phylogenetic genomesGerald Schönknecht et al., 2013, Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote, Science, http://www.sciencemag.org/content/339/6124/1207.long . A research group sequenced the 3.7-megabase genome of G. sulphuraria and compared this sequence information to the BLAST database. Their findings provide strong evidence that the extremophilic adaptations of G. sulphuraria are the result of horizontal gene transfer from a variety of archaea and bacteria. They argue that 5% of G. sulphuraria coding genes are result of horizontal gene transfers, and that many of these genes then "expanded" following transfer from bacteria and archaea. These findings build upon previous evidence that a non-linear gene pool has facilitated the unique environmental adaptations of the G. sulphuraria eukaryoteGerald Schönknecht et al., 2013, Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote, Science, http://www.sciencemag.org/content/339/6124/1207.long. Intron Evidence: G. sulphuraria is of the sub-phylum Cyanidiophyceae. The only other member of the Cyanidiophyceae, for which full-genome data are available is the Cyanidioschyzon merolae, which diverged from G. sulphuraria around 1 billion years ago. C. merolae is exclusively photoautotrophic and cannot live in high salt or heat environments. The C. merolae shares some genomic and phenotypic similarities with G. sulphuraria, although not very much. G. sulphuraria and C. merolae have only 42% orthologous genomes, and just 25% syntetic sections of blocks. The critical evolutionary evidence is found in the introns, which are often considered the hallmark of eukaryotic genes. The C. merolae genome has an average of 2 introns per gene, while the G. sulphuraria genome has an average of 0.8 introns per gene. Additionally, the G. sulphuraria genome has an average distance between coding genes of 22 bp. This highly condensed genome of G. sulphuraria is unique amongst eukaryotes, and is more reminiscent of a bacterial, or archaea genomeGerald Schönknecht et al., 2013, Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote, Science, http://www.sciencemag.org/content/339/6124/1207.long. Horizontal Transfer of Arsenic Resistance Genes: The relatively unique ability to live in harsh volcanic and acidic environments with high concentrations of metals and metaloids, such as arsenic, is generally a trait associated with bacteria and archaea. Bacteria are able to neutralize pump these harmful molecules via reductase enzymes and membrane active transporters Upon a genome-wide analysis of G. sulphuraria it was found that 5.2% of it's genes encode membrane transport proteins. Based on this finding, a most parsimonious phylogenetic tree for transport pumping proteins was constructed. It was found that G. sulphuraria was more closely related to bacteria than the other eukaryote trees. Of note, two of the intronless G. sulphuraria genes are almost identical to the bacterial arsenic-proton membrane pump, called "ArsB" found in thermoacidophilic bacteria. Extremophilic proteobacteria have an enzyme called "Mercuric Reductase", which reduces cytotoxic Hg2+ into mercury. An almost identical gene product is found in G. sulphuraria. The authors also belief this gene to have been horizontally acquiredGerald Schönknecht et al., 2013, Gene Transfer from Bacteria and Archaea Facilitated Evolution of an Extremophilic Eukaryote, Science, http://www.sciencemag.org/content/339/6124/1207.long. References: